The present invention relates to a rectifier circuit, in particular a rectifier circuit having a transformer, which has a primary side and a secondary side, the primary side having a first and a second input terminal for coupling in an AC voltage signal, and the secondary side comprising two secondary windings which are connected to one another and to a first output terminal, the first and the second secondary winding being connected by their other terminal to a first terminal of a first and a second diode disposed in the forward direction, and the second terminal of the first and of the second diode being connected to one another and to a second output terminal, it being possible for a load to be coupled between the first and second output terminals, the first and the second diode being realized by a first and a second MOSFET at least within a predetermined time range, by virtue of the fact that each MOSFET has a main current path with a main current direction HS and an auxiliary current pathxe2x80x94disposed parallel to the main current pathxe2x80x94with an auxiliary current direction NS and with a body diode disposed therein, the main current direction HS and the auxiliary current direction NS running opposite to one another, and it being possible for the first and the second MOSFET to be operated in such a way that the first and the second body diode realize the first and the second diode within the predetermined time range.
The literature discloses a rectifier circuit designated as single-phase center-tap connection. Rectifier circuits of this type are used, for example, to feed long rail and cable systems with electronic transformers. In this case, they serve for rectifying and smoothing the high-frequency output voltage. Without these two measures, the voltage drop along the line and the radio interference would be too high.
In a rectifier circuit of this type, the diodes required for rectification are often replaced by MOSFETs, in this case specifically by the body diode present in each MOSFET. FIG. 1 illustrates a MOSFET by way of example, the three terminals being marked D for drain, G for gate and S for source. The body diode is designated by BD. The direction in which current normally flows through the MOSFET, the so-called main current direction HS, runs from drain to source. The auxiliary current direction NS is disposed parallel and runs oppositely to the main current direction HS, as indicated by the arrows. If, in this circuit, the gate is not driven by a signal, then the MOSFET, via its body diode BD, acts like a normal diode, i.e. it permits a current flow only in one direction, to be precise in the auxiliary current direction NS as defined above. The disadvantage of this rectifier circuit disclosed in the prior art consists in the high power loss. In order to be able to supply the lamps fitted to the rail and cable systems with sufficient power, very high currents flow in the rectifier circuit, for example in the range between 20 and 40 A. In the case of a current of 25 A, for example, a power loss of 17.5 W is therefore produced on the diode, given a voltage drop across the diode of 0.7 V.
The same problems arise in the case of electronic transformers for rail and cable systems, in which the rectification is realized using schottky diodes. As a consequence of the high power loss, either use only at low ambient temperatures is considered or a fan has to be used for cooling, the service life of said fan being highly limited and being between 10,000 and 20,000 hours.
Taking this prior art as a departure point, the object of the present invention therefore consists in developing a rectifier circuit of the type mentioned in the introduction in such a way that the power loss is considerably reduced.
This object is achieved according to the invention by virtue of the fact that a rectifier circuit of the generic type furthermore comprises a first and a second drive circuit, which are designed to drive the first and the second MOSFET with regard to the current flow in the main current path in such a way that the current flow in the main current path is effected in the auxiliary current direction NS.
The invention is based on the insight that, in order to realize a diode function, not only can the body diode of a MOSFET which lies in the auxiliary current direction NS be used, but also, by active driving in a suitable manner, the main current path of a MOSFET. It is furthermore based on the insight that MOSFETs can also be operated inversely with regard to their main current direction HS, which runs from the drain to the source, without incurring damage. In this case, the driving for inverse operation corresponds to the driving during forward operation, i.e. the crucial voltage is the voltage present across the MOSFET between gate and source. With the MOSFET being switched into the on state by suitable driving of the gate, the current flowing through it therefore only has to overcome a very low forward resistance, which is of the order of magnitude of 6 mxcexa9. For the current of 25 A assumed by way of example above, a power loss of just 3.7 W is produced as a result. This low power loss makes it possible to operate a rectifier circuit according to the invention even at high ambient temperatures without using a fan.
In a preferred embodiment, the first and the second drive circuit can be operated in such a way that the current flow through the respective MOSFET is effected in the main current path in the auxiliary current direction NS at least in a first time range and in the auxiliary current path in the auxiliary current direction NS in a second time range. This measure makes it possible for the diode function to be realized by the main current path in a first time rangexe2x80x94by suitable driving of the gatexe2x80x94and in a customary manner by the body diode of the MOSFET in a second time range. The realization of the diode function by the body diode should, of course, be considered with regard to the reduction of the power loss only when the total current flowing through the MOSFET has a very small value or the time range in which the current flows is very short and hence the power loss of this current on the body diode is very low. This is the case in particular in the event of commutation of the current from one transistor to the other.
Furthermore, it is preferably provided that the first and the second drive circuit can be operated in such a way that no current flow at all takes place through the respective MOSFET at least in a third time range.
In a particularly advantageous embodiment, the first and the second drive circuit comprise a first and a second auxiliary winding connected to the transformer. This makes it possible to generate the drive signals for the two MOSFETs in conjunction with a very low outlay. When a large current is transformed by the transformer, the auxiliary windings generate a large signal, which is used for driving the gate electrode of the respective MOSFET. With this large signal, the MOSFETs can be switched without any difficulty into the on state, which is why the current which is actually flowing through the MOSFETs exactly at this moment traverses the MOSFET in the main current path, but in the auxiliary current direction NS. If only a very small current is transformed by the transformer, the signal obtained via the auxiliary windings does not suffice, under certain circumstances, to switch the respective MOSFET into the on state by driving of the respective gate electrode. This is entirely unproblematic in the present case, however, since it is exactly then that the current is also small which now flows through the respective MOSFET in the auxiliary current path in the auxiliary current direction NS and thus only a low power loss is produced on the body diode. If the two auxiliary windings are, moreover, assigned to the secondary side in terms of potential, the DC isolation between primary side and secondary side is realized in a particularly cost-effective and simple manner in this way. As an alternative, however, provision may also be made for the signals required for driving the MOSFETs to be generated on the primary side of the transformer by means of corresponding logic gates and to be transferred to the secondary side via very fast optocouplers or so-called SELV (Security Extra Low Voltage) transformers. When optocouplers are used, it is preferred to build up a supply voltage on the secondary side from the output voltage, which supply voltage supplies the respective drive circuit.
The respective drive circuit preferably comprises a respective modification network disposed between the respective auxiliary winding and the respective MOSFET and serving for modifying the output signal of the respective auxiliary winding. In particular, the effect that can be achieved with a modification network of this type is that the voltage provided by the auxiliary windings rises nonlinearly in order to make available, even when transforming low currents, a voltage which suffices to switch the respective MOSFET into the on state. Furthermore, the respective modification network should provide voltage limiting in order that an excessively high voltage is not applied to the gate of the respective MOSFET, which might lead to the destruction thereof. In particular, it is preferred to dimension the modification network in such a way that the voltage to be coupled to the gate of the respective MOSFET rises rapidly above 10 V in a non-linear manner and is limited at 18 V.
In accordance with a particularly preferred embodiment, the first and the second modification network are designed in such a way that, during a commutation of the current from the first MOSFET to the second, and vice versa, it is always the case that at least one of the two MOSFETs is turned on. This measure ensures that even in the so-called dead time, the current flow is not effected via the auxiliary current path, i.e. via the body diode, but rather via the main current path. In other words, even during the dead time, the power loss incurred is merely the product of forward resistance of a MOSFET and current flowing through it.
This is preferably achieved in that the first and the second modification network are designed in such a way that a changeover of the first and second MOSFETs from the on to the off state and/or vice versa is essentially effected in two stages, the first stage being correlated with the commutation from the on to the off state, while the second stage is correlated with the on or off state of the respective MOSFET.
Further advantageous embodiments emerge from the subclaims.